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Merge pull request #201 from aaronsb/fix/190-client-driven-reinit-signal

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fix(mcp): client-driven session re-init via spec 404 (ADR-106, closes #190)
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1
docs/architecture/INDEX.md
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@ -26,6 +26,7 @@ _Server architecture, transport, connection handling, plugin lifecycle_
| [ADR-103](./core/ADR-103-tls-certificate-and-key-storage-strategy-for-localhost-https.md) | TLS certificate and key storage strategy for localhost HTTPS | Accepted |
| [ADR-104](./core/ADR-104-offload-cpu-bound-semantic-operations-to-worker-threads-and-deconflict-the-sse-route.md) | Offload CPU-bound semantic operations to worker threads and deconflict the SSE route | Accepted |
| [ADR-105](./core/ADR-105-remove-dormant-worker-offload-path-partial-reversal-of-adr-104.md) | Remove dormant worker-offload path (partial reversal of ADR-104) | Accepted |
| [ADR-106](./core/ADR-106-client-driven-session-re-initialization-via-spec-compliant-http-404.md) | Client-driven session re-initialization via spec-compliant HTTP 404 | Accepted |
## Tools & API
_MCP tool design, semantic operations, graph operations, formatters_

148
docs/architecture/core/ADR-106-client-driven-session-re-initialization-via-spec-compliant-http-404.md Normal file
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@ -0,0 +1,148 @@
---
status: Accepted
date: 2026-05-18
deciders:
- aaronsb
- claude
related: []
---
# ADR-106: Client-driven session re-initialization via spec-compliant HTTP 404
## Context
After ~3h idle, the MCP session is silently dropped and **every** subsequent
tool call fails with HTTP 400 `"Bad Request: Server not initialized"`,
unrecoverable without restarting the MCP client (#128, user-facing symptom;
#190, the engineering investigation). The root cause was proven against
`@modelcontextprotocol/sdk@1.29.0` (the version on `main`) and re-verified
during this work:
When a request arrived bearing an `Mcp-Session-Id` the server no longer held
a transport for (session evicted), the handler tried a **server-side
synthetic `initialize`**: it built `compatReq = { ...req, headers }` — a
plain object, not a `stream.Readable` — and fed it to SDK 1.29's
`StreamableHTTPServerTransport.handleRequest`. SDK 1.29 delegates to
`@hono/node-server`'s `getRequestListener`, whose `Readable.toWeb(incoming)`
cannot consume a non-stream; even with a real `IncomingMessage`,
`getRequestListener` throws `RequestError("Missing host header")` → 400, and
an `accept` header set post-construction does not survive hono's lazy Web
`Request` reconstruction → SDK 406. **No synthetic-Node-object path reaches
`_initialized = true` on SDK 1.29.** The synthetic init always failed,
"failed open," and fell through to the 400. Two community PRs (#147 real
`IncomingMessage`; #150 retry-with-backoff) were reviewed and declined: the
failure is *deterministic*, not transient, and operates at the wrong layer.
The Streamable HTTP transport spec already defines the correct recovery
mechanism (Session Management):
- **§3** — the server **MAY** terminate a session at any time, after which
it **MUST** respond to requests carrying that session ID with **HTTP 404**.
- **§4** — a client receiving **HTTP 404** for a request bearing an
`Mcp-Session-Id` **MUST** start a new session by sending a fresh
`InitializeRequest` with no session ID.
- **§2** — a non-initialize request with no `Mcp-Session-Id` **SHOULD** get
**HTTP 400**.
The server was doing none of this for the evicted-session case; it
fabricated a phantom transport and returned a non-spec
`400 -32000 "Server not initialized"`, which no client treats as a
session-expiry signal — hence the unrecoverable loop.
## Decision
**Stop attempting server-side synthetic initialization. Emit the spec's
session-lifecycle signal and let the client re-initialize itself
(client-driven re-init).**
In `MCPHttpServer.handleMCPRequest`:
- Keep the three transport-binding paths unchanged: existing live session;
recreate-on-`initialize` for a known session ID; fresh `initialize` with
no session ID.
- For a non-`initialize` request whose `Mcp-Session-Id` has no live
transport (evicted/stale): respond **HTTP 404** with the
`Mcp-Session-Id` echoed and a courtesy JSON-RPC error body
(`code: -32001`, message instructing re-initialize). Spec §3.
- For a non-`initialize` request with no `Mcp-Session-Id`: respond
**HTTP 400**. Spec §2.
- Delete the synthetic compat-initialize block, the `requireInitializeNotice`
machinery, the `createNullRes`/`NullResponse` shim, and the phantom
transport creation. A spec-compliant client/bridge re-initializes on the
404 and the **next** request opens a fresh session normally — no client
restart (fixes #128).
The HTTP status is the load-bearing signal; the JSON-RPC body is courtesy.
`DELETE /mcp` (spec §5) is already handled and is left as-is.
### Acceptance (restated for this direction)
#190's original acceptance ("non-initialize request returns 2xx") was
written before a direction was chosen and described the *server-heal*
candidate. For client-driven re-init the equivalent recovery is:
> A non-`initialize` request bearing an evicted `Mcp-Session-Id` returns
> **HTTP 404 with the `Mcp-Session-Id` header set** (spec §3 signal), **and**
> a subsequent `initialize` request with no session ID returns **2xx with a
> fresh `Mcp-Session-Id`** (a working new session). Together the client
> recovers without a restart, per spec §4. Asserted by a harness driving the
> request pair through the handler against the SDK version on `main`.
### Known client caveat
Spec §4 is the client's obligation. `mcp-remote` does not "clear session +
reinit" explicitly; on a 404 it recurses through `connectToRemoteServer`
with a fresh transport, whose first message is an `initialize` with no
session ID — the same observable outcome. Some clients (e.g. certain Claude
Code builds) reportedly do not honor §4 at all. That is an upstream client
gap, not something the server can paper over without re-introducing the
proven-broken synthetic-init class. The server's contract is the spec;
end-to-end recovery is validated by functional test against the real
client/bridge.
## Consequences
### Positive
- #128 becomes recoverable without a client restart on spec-compliant
clients/bridges; the unrecoverable 400 loop is gone.
- Deletes a large block of proven-broken, SDK-version-fragile synthetic-init
code and its shims; the handler now expresses exactly the spec's state
machine.
- No dependency on SDK internals or hono request reconstruction — robust
across SDK upgrades.
### Negative
- Recovery depends on the client honoring spec §4. Clients that don't will
still need a manual reconnect — but they did before too, and the server is
now spec-correct rather than emitting a misleading non-spec 400.
Mitigation: validated by functional test against the actual
Claude/`mcp-remote` path.
### Neutral
- Requires a functional re-test against a real client (BRAT prerelease →
reconnect) to confirm end-to-end recovery; covered by the project's
prerelease loop.
- A deterministic regression harness drives the synthetic request pair
through `handleMCPRequest` and asserts the 404+header / 2xx-reinit pair;
added with this change.
## Alternatives Considered
- **#147 — real synthetic `IncomingMessage` round-tripped through hono.**
Rejected: still cannot reach `_initialized` on SDK 1.29 (RequestError →
400, then 406); right instinct, wrong layer (proven empirically).
- **#150 — retry the synthetic init with backoff.** Rejected: the failure
is deterministic, not transient; retry only adds latency before the
identical failure.
- **Cosmetic status rewrite (`-32001 "session expired"`).** Rejected: a
body relabel is not a recovery; clients still don't re-init and the loop
persists. Explicitly called out as a non-fix in #190.
- **Server-side session keep-alive / TTL extension to prevent the drop.**
Rejected as the primary fix: it narrows the window but does not make a
*dropped* session recoverable (network loss, restart, eviction under
pressure still strand the client). Orthogonal; may be revisited
separately. The spec's 404/re-init path is the designed recovery and is
needed regardless.

223
src/mcp-server.ts
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@ -58,22 +58,6 @@ interface ServerWithTimeouts {
setTimeout: (msecs: number) => unknown;
}
/** Null response shim for internal initialize calls */
interface NullResponse {
writeHead: (status: number, headers?: Record<string, string>) => void;
setHeader: (k: string, v: string) => void;
getHeader: (k: string) => string | undefined;
statusCode: number;
headersSent: boolean;
write: (chunk: unknown) => void;
end: (data?: unknown) => void;
on: (event: string, handler: (...args: unknown[]) => void) => void;
once: (event: string, handler: (...args: unknown[]) => void) => void;
status: (v: number) => NullResponse;
json: (body: unknown) => void;
send: (body?: unknown) => void;
}
/** Connection pool stats response */
interface ConnectionPoolStatsResponse {
enabled: boolean;
@ -104,7 +88,10 @@ export class MCPHttpServer {
private connectionCount: number = 0;
private plugin?: MCPPluginRef; // Reference to the plugin
private connectionPool?: ConnectionPool;
private sessionManager?: SessionManager;
// Assigned unconditionally in the constructor. Non-optional so the
// "initialize is never short-circuited as a terminated session" invariant
// (ADR-106) is guaranteed by the type system, not just construction order.
private sessionManager: SessionManager;
private certificateManager: CertificateManager | null;
private isHttps: boolean = false;
@ -380,6 +367,57 @@ export class MCPHttpServer {
});
}
/**
* Emit the Streamable HTTP spec's session-lifecycle signal so the client
* can recover a dropped/evicted session on its own (client-driven re-init,
* ADR-106).
*
* - If the request carried an `Mcp-Session-Id` we no longer hold a
* transport for, the session is terminated: respond **HTTP 404**
* (Session Management §3). A spec-compliant client/bridge MUST then
* start a new session by sending a fresh `InitializeRequest` with no
* session ID (§4) — no client restart required, fixing #128.
* - If no `Mcp-Session-Id` was sent on a non-initialize request, a session
* is required: respond **HTTP 400** (§2).
*
* The HTTP status is the load-bearing signal; the JSON-RPC error body is
* courtesy for clients that surface it. We deliberately do not attempt a
* server-side synthetic initialize — that cannot drive SDK 1.29's
* web-standard transport to an initialized state (see #190).
*/
private sendSessionTerminated(
res: express.Response,
request: JsonRpcRequest | undefined,
sessionId: string | undefined
): void {
const id = request?.id ?? null;
if (sessionId) {
// Spec §3: terminated session → 404; client re-inits per §4.
res.setHeader('Mcp-Session-Id', sessionId);
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32001,
message: 'Session expired or not found. Start a new session by sending an initialize request without a session ID.',
data: { sessionId }
},
id
});
Debug.log(`🔁 Session ${sessionId} terminated → 404 (client should re-initialize per MCP spec §4)`);
return;
}
// Spec §2: session required for non-initialize requests → 400.
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32600,
message: 'Bad Request: a session is required. Send an initialize request first.'
},
id
});
Debug.log('⚠️ Non-initialize request with no session id → 400 (session required)');
}
private async handleMCPRequest(req: express.Request, res: express.Response): Promise<void> {
try {
const request = req.body as JsonRpcRequest | undefined;
@ -405,35 +443,6 @@ export class MCPHttpServer {
}
let mcpServer: MCPServer;
// Helper: create a null response shim so we can send an internal initialize
const createNullRes = (): NullResponse => {
const headers: Record<string, string> = {};
let sent = false;
let code = 200;
const resp: NullResponse = {
// Node-style response interface (minimal no-op implementation)
writeHead: (_status: number, _headers?: Record<string, string>) => { code = _status; sent = true; },
setHeader: (k: string, v: string) => { headers[k] = v; },
getHeader: (k: string) => headers[k],
get statusCode() { return code; },
set statusCode(v: number) { code = v; },
get headersSent() { return sent; },
write: (_chunk: unknown) => { /* no-op */ },
end: (_data?: unknown) => { sent = true; },
on: (_event: string, _handler: (...args: unknown[]) => void) => { /* no-op */ },
once: (_event: string, _handler: (...args: unknown[]) => void) => { /* no-op */ },
// Express-like helpers (used by some wrappers)
status: (v: number) => { code = v; return resp; },
json: (_body: unknown) => { sent = true; },
send: (_body?: unknown) => { sent = true; },
};
return resp;
};
// When a non-initialize request arrives without an active transport,
// we return a JSON-RPC error instructing the client to initialize using
// the provided session ID. This avoids fragile internal auto-initialize.
let requireInitializeNotice = false;
// Helper: register transport with lifecycle hooks
const attachTransportHandlers = (sessId: string, tr: StreamableHTTPServerTransport) => {
try {
@ -488,18 +497,18 @@ export class MCPHttpServer {
this.connectionCount++;
Debug.log(`♻️ Recreated transport for session ${sessionId} (requests: ${session.requestCount})`);
} else {
// Create transport and require client initialize on next call
const newSessionId = sessionId; // reuse provided id (guaranteed by branch)
mcpServer = this.mcpServerPool.getOrCreateServer(newSessionId);
effectiveSessionId = newSessionId;
transport = new StreamableHTTPServerTransport({
sessionIdGenerator: () => effectiveSessionId
});
await mcpServer.connect(transport);
this.transports.set(effectiveSessionId, transport);
attachTransportHandlers(effectiveSessionId, transport);
this.connectionCount++;
requireInitializeNotice = true;
// Stale/evicted session: the client presented an Mcp-Session-Id
// we no longer hold a transport for, and this is not an
// initialize request. Per the Streamable HTTP spec (Session
// Management §3) the server MUST respond HTTP 404 for a
// terminated session; per §4 the client must then start a new
// session by sending a fresh InitializeRequest with no session
// ID. We do NOT fabricate a transport or attempt a server-side
// synthetic initialize — that cannot drive SDK 1.29's
// web-standard transport to an initialized state (ADR-106 /
// #190) and only produces an unrecoverable 400 loop (#128).
this.sendSessionTerminated(res, request, sessionId);
return;
}
} else if (!sessionId && isInitializeRequest(request)) {
// New initialization request - create new transport with session
@ -525,91 +534,25 @@ export class MCPHttpServer {
this.sessionManager.getOrCreateSession(effectiveSessionId);
}
} else {
// No or unknown session on non-initialize request.
// Generate a session (or reuse provided) and require client initialize next.
const newSessionId = sessionId ?? randomUUID();
mcpServer = this.mcpServerPool.getOrCreateServer(newSessionId);
effectiveSessionId = newSessionId;
transport = new StreamableHTTPServerTransport({
sessionIdGenerator: () => effectiveSessionId
});
await mcpServer.connect(transport);
this.transports.set(effectiveSessionId, transport);
attachTransportHandlers(effectiveSessionId, transport);
this.connectionCount++;
requireInitializeNotice = true;
// Non-initialize request with no usable session. Either:
// - an Mcp-Session-Id we don't hold a transport for → spec §3
// terminated-session signal (HTTP 404), client re-inits per §4;
// - no Mcp-Session-Id at all and not an initialize → spec §2
// "session required" (HTTP 400).
// sendSessionTerminated picks the status from sessionId presence.
// No phantom transport, no synthetic initialize (see #190/#128).
this.sendSessionTerminated(res, request, sessionId);
return;
}
// Compatibility: if we just created a transport for a non-initialize call,
// attempt a safe, internal initialize to avoid client retry loops.
if (requireInitializeNotice && transport && !isInitializeRequest(request)) {
// Clone req to ensure session header is present for compat initialize
const compatReq = {
...req,
headers: {
...req.headers,
'mcp-session-id': effectiveSessionId
}
} as unknown as IncomingMessage;
const versionsToTry = ['2025-06-18', '2024-11-05', '1.0'];
let initOk = false;
for (const ver of versionsToTry) {
try {
const initReq = {
jsonrpc: '2.0',
id: '__compat_init__',
method: 'initialize',
params: {
protocolVersion: ver,
capabilities: {},
clientInfo: { name: 'obsidian-mcp-compat', version: getVersion() }
}
} as unknown;
await transport.handleRequest(compatReq, createNullRes() as unknown as ServerResponse, initReq);
initOk = true;
Debug.log(`Compat initialize succeeded with protocolVersion=${ver}`);
break;
} catch (e) {
Debug.error(`Compat initialize attempt failed (protocolVersion=${ver}):`, e);
}
}
// Fail-open: even if initialize failed, proceed with the original request
// to avoid client retry loops. The transport/server may still reject it,
// but this gives us a concrete server error to act on.
requireInitializeNotice = false;
if (!initOk) {
Debug.log('Compat initialize failed; proceeding without explicit initialize (fail-open).');
}
}
// If initialization is still required and this isn't an initialize request,
// instruct the client to initialize for this session.
if (requireInitializeNotice && !isInitializeRequest(request)) {
const id = request?.id ?? null;
res.setHeader('Mcp-Session-Id', effectiveSessionId);
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32000,
message: 'Bad Request: Server not initialized',
data: { sessionId: effectiveSessionId }
},
id
});
return;
}
// Safety: ensure we have a transport before forwarding
// Safety: every reachable path above either bound a live `transport`
// (existing session, recreate-on-initialize, fresh initialize) or
// returned a spec-compliant terminated/required-session response. A
// missing transport here is an unexpected invariant break, not a stale
// session — surface it explicitly rather than papering it.
if (!transport) {
const id = request?.id ?? null;
if (effectiveSessionId) {
res.setHeader('Mcp-Session-Id', effectiveSessionId);
}
res.status(200).json({
jsonrpc: '2.0',
error: { code: -32001, message: 'No active transport/session. Please initialize and retry.', data: effectiveSessionId ? { sessionId: effectiveSessionId } : undefined },
id
});
Debug.error('Invariant: no transport after session resolution');
this.sendSessionTerminated(res, request, sessionId);
return;
}

134
tests/mcp-session-reinit.test.ts Normal file
View file

@ -0,0 +1,134 @@
/**
* Regression harness for #190 / #128 (ADR-106): client-driven session
* re-initialization via the spec-compliant HTTP 404 signal.
*
* The pre-ADR-106 server tried a server-side synthetic `initialize` for an
* evicted session, which cannot drive SDK 1.29's web-standard transport to an
* initialized state, "failed open," and returned a non-spec
* `400 -32000 "Server not initialized"` that no client treats as a
* session-expiry signal — an unrecoverable loop until client restart.
*
* ADR-106: a non-`initialize` request bearing an evicted `Mcp-Session-Id`
* must return HTTP 404 with the `Mcp-Session-Id` echoed (spec §3) so a
* compliant client/bridge re-initializes per §4; a non-`initialize` request
* with no session id must return HTTP 400 (spec §2); an `initialize` request
* must never be short-circuited as "terminated".
*
* These paths return before any SDK transport is constructed, so they are
* deterministic without exercising the SDK transport in jsdom (the exact
* thing #190 established cannot be driven synthetically).
*/
import { MCPHttpServer } from '../src/mcp-server';
import { App } from 'obsidian';
jest.mock('fs', () => ({
existsSync: jest.fn(() => false),
mkdirSync: jest.fn(),
readFileSync: jest.fn(),
writeFileSync: jest.fn()
}));
interface MockRes {
statusCode: number;
headers: Record<string, string>;
body: unknown;
headersSent: boolean;
setHeader: (k: string, v: string) => void;
status: (c: number) => MockRes;
json: (b: unknown) => void;
}
function makeRes(): MockRes {
const res: MockRes = {
statusCode: 0,
headers: {},
body: undefined,
headersSent: false,
setHeader(k, v) { this.headers[k.toLowerCase()] = v; },
status(c) { this.statusCode = c; return this; },
json(b) { this.body = b; this.headersSent = true; }
};
return res;
}
function makeReq(body: unknown, sessionId?: string) {
return {
body,
headers: sessionId ? { 'mcp-session-id': sessionId } : {}
};
}
describe('MCP session re-initialization signal (ADR-106 / #190 / #128)', () => {
let server: MCPHttpServer;
beforeEach(() => {
const mockApp = new App();
mockApp.vault = {
...mockApp.vault,
adapter: { basePath: '/mock/vault/path' }
} as any;
server = new MCPHttpServer(mockApp, 3001);
});
test('evicted session + non-initialize request → HTTP 404 with Mcp-Session-Id echoed (spec §3)', async () => {
const res = makeRes();
const staleSession = 'stale-session-id-1234';
// No transport is registered for this id → evicted/terminated.
await (server as any).handleMCPRequest(
makeReq({ jsonrpc: '2.0', id: 7, method: 'tools/call', params: {} }, staleSession),
res
);
expect(res.statusCode).toBe(404);
expect(res.headers['mcp-session-id']).toBe(staleSession);
// Must NOT be the old unrecoverable non-spec signal.
expect(res.statusCode).not.toBe(400);
const body = res.body as { error?: { code?: number; message?: string } };
expect(body.error?.code).toBe(-32001);
expect(body.error?.message).toMatch(/initialize/i);
});
test('non-initialize request with no session id → HTTP 400 (spec §2)', async () => {
const res = makeRes();
await (server as any).handleMCPRequest(
makeReq({ jsonrpc: '2.0', id: 8, method: 'tools/call', params: {} }),
res
);
expect(res.statusCode).toBe(400);
expect(res.headers['mcp-session-id']).toBeUndefined();
});
test('initialize request is never short-circuited as a terminated session', async () => {
const spy = jest.spyOn(server as any, 'sendSessionTerminated');
// (a) initialize with no session id (fresh) and (b) initialize bearing a
// stale session id (recreate path) must both bypass the 404/400 signal.
for (const sid of [undefined, 'previously-evicted-id']) {
const res = makeRes();
await (server as any).handleMCPRequest(
makeReq(
{ jsonrpc: '2.0', id: 1, method: 'initialize', params: { protocolVersion: '2025-06-18', capabilities: {}, clientInfo: { name: 't', version: '0' } } },
sid
),
res
);
expect(res.statusCode).not.toBe(404);
}
expect(spy).not.toHaveBeenCalled();
spy.mockRestore();
});
test('sendSessionTerminated emits the spec signals directly', () => {
const withId = makeRes();
(server as any).sendSessionTerminated(withId, { id: 3 }, 'sess-abc');
expect(withId.statusCode).toBe(404);
expect(withId.headers['mcp-session-id']).toBe('sess-abc');
expect((withId.body as { id?: unknown }).id).toBe(3);
const noId = makeRes();
(server as any).sendSessionTerminated(noId, { id: null }, undefined);
expect(noId.statusCode).toBe(400);
expect(noId.headers['mcp-session-id']).toBeUndefined();
});
});